Double silver low-emissivity and solar control coatings

ABSTRACT

A low-emissivity multilayer coating includes, in order outward from the substrate, a first layer including a layer containing titanium oxide, a layer containing silicon nitride, or a sublayer layer containing titanium oxide in combination with a sublayer containing silicon nitride; a second layer including Ag; a third layer including at least one layer selected from titanium oxide layers and silicon nitride layers; a fourth layer including Ag; and a fifth layer including silicon nitride. The color of the coatings can be varied over a wide range by controlling the thicknesses of the layers of titanium oxide, silicon nitride and Ag. A diffusion barrier of oxidized metal protects relatively thin, high electrical conductivity, pinhole free Ag films grown preferentially on zinc oxide substrates. Oxygen and/or nitrogen in the Ag films improves the thermal and mechanical stability of the Ag. Dividing the first layer of titanium oxide, the Ag layers, and/or the third layer with a sublayer of oxidized metal can provide greater thermal and mechanical stability to the respective layers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to low emissivity coatings. Morespecifically, the present invention relates to multilayer coatings forcontrolling thermal radiation from substrates transparent to visiblelight.

2. Discussion of the Background

Solar control coatings on transparent panels or substrates are designedto permit the passage of visible light while blocking infrared (IR)radiation. High visible transmittance, low emissivity coatings on, e.g.,architectural glass and automobile windows can lead to substantialsavings in costs associated with environmental control, such as heatingand cooling costs.

Generally speaking, coatings that provide for high visible transmittanceand low emissivity are made up of a stack of films. The stack includesone or more thin metallic films, with high IR reflectance and lowtransmissivity, disposed between anti-reflective dielectric layers. TheIR reflective metallic films may be virtually any reflective metal, suchas silver, copper, or gold. Silver (Ag) is most frequently used for thisapplication due to its relatively neutral color. The anti-reflectivedielectric layers are generally metal oxides selected to minimizevisible reflectance and enhance visible transmittance.

Conventional low emissivity coatings generally strive to maintainreflection relatively constant throughout the visible spectrum so thatthe coating has a “neutral” color; i.e., is essentially colorless.However, conventional low-emissivity coatings fail to provide theextremes of reflected color required for aesthetic and other reasons bycertain applications.

To achieve the desired properties in a coated substrate, the compositionand thickness of each of the layers of a multilayer coating must bechosen carefully. For example, the thickness of an IR reflective layersuch as Ag must be chosen carefully. It is well known that theemissivity of a Ag film tends to decrease with decreasing Ag sheetresistance. Thus, to obtain a low emissivity Ag film, the sheetresistance of the Ag film should be as low as possible. Because filmsurfaces and pinholes in very thin Ag films contribute to sheetresistance, increasing Ag film thickness to separate film surfaces andeliminate pinholes can decrease sheet resistance. However, increasing Agfilm thickness will also cause visible transmission to decrease. Itwould be desirable to be able to increase visible transmission bydecreasing Ag film thickness without increasing sheet resistance andemissivity.

Thin, transparent metal films of Ag are susceptible to corrosion (e.g.,staining) when they are brought into contact, under moist or wetconditions, with various staining agents, such as atmosphere-carriedchlorides, sulfides, sulfur dioxide and the like. To protect the Aglayers, various barrier layers can be deposited on the Ag. However, theprotection provided by conventional barrier layers is frequentlyinadequate.

Coated glass is used in a number of applications where the coating isexposed to elevated temperatures. For example, coatings on glass windowsin self-cleaning kitchen ovens are repeatedly raised to cookingtemperatures of 120-230° C., with frequent excursions to, e.g., 480° C.during cleaning cycles. In addition, when coated glass is tempered orbent, the coating is heated along with the glass to temperatures on theorder of 600° C. and above for periods of time up to several minutes.These thermal treatments can cause the optical properties of Ag coatingsto deteriorate irreversibly. This deterioration can result fromoxidation of the Ag by oxygen diffusing across layers above and belowthe Ag. The deterioration can also result from reaction of the Ag withalkaline ions, such as sodium (Na+), migrating from the glass. Thediffusion of the oxygen or alkaline ions can be facilitated andamplified by the deterioration or structural modification of thedielectric layers above and below the Ag. Coatings must be able towithstand these elevated temperatures. However, multilayer coatingsemploying Ag as an infrared reflective film frequently cannot withstandsuch temperatures without some deterioration of the Ag film.

It would be desirable to provide low emissivity, multilayer coatingsexhibiting any of a wide range of colors, along with improved chemical,thermal and mechanical stability.

SUMMARY OF THE INVENTION

The present invention provides multilayer coatings that can reduce theinfrared emissivity of a substrate with minimal reduction in visibletransmittance. The inventive coatings can be designed to exhibit any ofa wide variety of different colors in reflection.

The multilayer coating includes, in numerical order outward from thesubstrate, a first layer including a layer containing titanium oxide, alayer containing silicon nitride, or a superlattice of one or moresublayer containing titanium oxide in combination with one or moresublayer containing silicon nitride; a second layer including Ag; athird layer including at least one layer selected from titanium oxidelayers and silicon nitride layers; a fourth layer including Ag; and afifth layer including silicon nitride. By varying the thicknesses of thelayers of titanium oxide and silicon nitride the reflected color of thecoating can be “tuned” within any one of the four color coordinatequadrants in the CIE L*a*b* color space.

When the first layer is amorphous titanium oxide, the first layer isparticularly dense and provides exceptional barrier properties againstoxygen and alkaline ions migrating from the substrate. In addition,amorphous titanium oxide provides an extremely smooth surface, whichaids in the deposition of thinner pin-hole free Ag films with loweremissivity and higher visible transmission in the second and fourthlayers.

The second and fourth layers can include a sublayer of zinc oxide,serving as a substrate for a sublayer of the Ag, and additionally asublayer of oxidized metal deposited on the Ag sub-layer. The zinc oxideprovides a substrate on which relatively thin, high electricalconductivity, Ag films preferentially grow. The sublayer of oxidizedmetal protects the Ag by acting as a diffusion barrier against oxygen,water and other reactive atmospheric gases, and also improves adhesion.

Incorporating oxygen and/or nitrogen into the Ag sublayers of the secondand fourth layers can improve the strength and mechanical stability ofthe Ag sublayers.

Dividing a first layer of titanium oxide and/or silicon nitride, the Agsublayers, and/or the third layer with a sublayer of oxidized metal canprovide greater strength and mechanical stability to the divided layersduring heat treatments.

The fifth layer of silicon nitride provides enhanced resistance toscratching.

In embodiments, multilayer coatings according to the present inventioncan undergo heat treatments, suitable to temper or bend glass, withminimal mechanical or optical degradation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows bright field transmission electron micrographs comparing Agdeposited directly on amorphous TiO_(x) with Ag deposited directly onZnO (5 nm thick) resting on amorphous TiO_(x). In both cases theamorphous TiO_(x) was deposited on 50 nm thick, amorphous siliconnitride membranes.

FIG. 2 shows dark field transmission electron micrographs comparing Agdeposited directly on amorphous TiO_(x) with Ag deposited directly onZnO (5 nm thick) resting on amorphous TiO_(x).

FIG. 3 is a transmission electron micrograph showing a discontinuouslayer of Ag, containing pinholes, deposited on amorphous TiO_(x).

FIG. 4 a shows CIE 1976 L*a*b* (CIELAB) transmitted color variationsfrom multilayer coatings on glass substrates resulting from changes inlayer thicknesses.

FIG. 4 b shows CIE 1976 L*a*b* (CIELAB) reflected glass side colorvariations from multilayer coatings on glass substrates resulting fromchanges in layer thicknesses.

FIG. 4 c shows CIE 1976 L*a*b* (CIELAB) reflected coating side colorvariations from multilayer coatings on glass substrates resulting fromchanges in layer thicknesses.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a low emissivity multilayer coating inwhich the color in reflection can be varied to lie in any of the fourcolor quadrants of the CIE L*a*b* color space. The coating can beprovided with a normal emissivity of 0.02≦∈≦0.10, a solar transmission(T_(sol)) of less than 45%, a solar reflection (R_(sol)) from either thecoating or glass substrate side of greater than 28%, and CIE 1931 Yxy(Chromaticity) transmission and reflection (from either the coating orglass side) greater than 75% and less than 7%, respectively.

An embodiment of the low-emissivity coating of the present inventionappears in Table 1:

TABLE 1 Layer Material 5 silicon nitride 4 Ag 3 titanium oxide; siliconnitride; or superlattice of titanium oxide and silicon nitride 2 Ag 1titanium oxide and/or silicon nitride layers 0 substrateThe coating is deposited on a substrate, and includes, in numericalorder outward from the substrate, a first layer including a layercontaining titanium oxide, a layer containing silicon nitride, or asuperlattice of one or more sublayer containing titanium oxide incombination with one or more sublayer containing silicon nitride; asecond layer including Ag; a third layer including at least one layerselected from titanium oxide layers and silicon nitride layers; a fourthlayer including Ag; and a fifth layer including silicon nitride. Themultiple layers of silver in the low emissivity coating of the presentinvention provide greater efficiency in reflecting IR radiation, and asharper cut-off between transmitted and reflected wavelengths, than ispossible with a single layer of silver.

Layer 0 is the substrate. The multilayer coating of the presentinvention is deposited on and is mechanically supported by thesubstrate. The substrate surface serves as a template for the coating,and influences the surface topography of the coating. To maximizetransmission of visible light, preferably the surface of the substratehas a roughness less than the wavelength of the light. Such a smoothsurface can be formed by, e.g., solidifying a melt of the substrate. Thesubstrate can be any material having an emissivity that can be loweredby the multilayer coating of the present invention. For architecturaland automotive applications, the substrate is preferably a materialwhich has superior structural properties and minimum absorption in thevisible and near-infrared spectra regions where the solar energy isconcentrated. Crystalline quartz, fused silica, soda-lime silicate glassand plastics, e.g., polycarbonates and acrylates, are all preferredsubstrate materials.

Layer 1 promotes adhesion between the coating and the substrate; servesas a barrier to oxygen and alkaline ions (e.g., Na⁺) migrating from thesubstrate to the coating; influences the surface roughness of thecoating; and promotes the transmission of visible light through thecoating. The present inventors have discovered that titanium oxide andsilicon nitride are both well suited to these functions.

Titanium oxide is particularly well suited for layer 1. The titaniumoxide is preferably a dielectric and electrically insulating. Thetitanium oxide of layer 1 can be TiO_(x), where x ranges from 1 to 2.The titanium oxide can be sputtered in a variety of phases: e.g., asrutile and anatase polycrystalline phases, and as an amorphous phase.Anatase and rutile layers provide higher indices of refraction, makingit possible to attain higher visible transmission. However, preferablythe titanium oxide is amorphous, because amorphous titanium oxide formsa denser layer than other metal oxides and provides a superior barrierto oxygen and alkaline ions diffusing from the substrate. In addition,because an amorphous layer of titanium oxide is smoother than apolycrystalline layer, the amorphous layer of titanium oxide permitsthinner continuous films of infrared reflective Ag to be deposited thandoes a polycrystalline film. An amorphous titanium oxide layer can beformed by DC, AC, or RF magnetron sputtering under conditions well knownin the art.

The silicon nitride of layer 1 can be SiN_(x), where x varies fromgreater than 0 to 1.34. When x=1.34 in SiN_(x), the silicon nitride isstoichiometric Si₃N₄.

The titanium oxide of layer 1 has a higher index of refraction(approximately 2.4 at 550 nm) compared with silicon nitride (greaterthan 1.9 at 550 nm) and many other oxides. Thus, the titanium oxidepromotes transmission and reduces reflection of light to a greaterextent than these other materials. As a result of titanium oxide'shigher index of refraction, a similar optical behavior in layer 1 can beachieved using a thinner layer of titanium oxide than of the othermaterials. Alternatively, by replacing a conventional oxide in layer 1with titanium oxide of equal thickness the thickness of subsequent IRreflective silver layers in a coating can be increased without reducingvisible transmittance of the coating.

When present in layer 1, the titanium oxide can have a thickness in therange of about 5 to 30 nm, preferably 5 to 20 nm, more preferably 5 to15 nm. If the titanium oxide film is less than 5 nm thick, then the filmfails to block migration of oxygen and alkaline ion impurities from thesubstrate. If the titanium oxide film is thicker than 30 nm, then thefilm tends to block transmission of visible light. Most preferably, thetitanium oxide of layer 1 is about 10 nm thick.

When present in layer 1, the silicon nitride can have a thickness in arange from 5 to 30 nm, preferably 5 to 20 nm, more preferably 5 to 15nm. The silicon nitride can enhance the barrier properties and alsoinfluence the optical properties of the coating when a sufficientthickness of silicon nitride is present.

Layer 2 is designed to reflect IR radiation. To accomplish this task,while retaining the possibility of a relatively neutral color inreflection, layer 2 is formed primarily from Ag. The Ag of layer 2 canhave a thickness in the range of about 8 to 16 nm, preferably 8 to 14nm, more preferably 10 to 14 nm, most preferably about 12 nm.

Layer 3 includes one or more anti-reflective layers to enhance visibletransmission. The anti-reflective layers are dielectric materials andelectrically insulating. Preferably, the dielectric materials areselected from titanium oxide and silicon nitride. The titanium oxide canbe TiO_(x), where x varies from greater than 1 to 2, and is preferablyamorphous. The silicon nitride can be SiN_(x), where x varies fromgreater than 0 to 1.34. When x=1.34 in SiN_(x), the silicon nitride isstoichiometric Si₃N₄. Preferably, layer 3 is Si₃N₄. Because titaniumoxide has a higher index of refraction than silicon nitride, the sameoptical behavior can be obtained using a thinner layer of titanium oxidethan silicon nitride. On the other hand, silicon nitride providesgreater mechanical stability than titanium oxide during heat treatments,and thus greater heat treatability. The combination of silicon nitridewith titanium oxide in a superlattice provides both the opticaladvantages of the higher index of refraction of titanium oxide and thethermal and mechanical stability advantages associated with siliconnitride. The higher average index of refraction of the titaniumoxide/silicon nitride superlattice relative to silicon nitride alonepermits a higher visible, photopic, transmission for the same Agthickness, or a similar photopic transmission for an increased number ofstabilizing barrier layers. The thickness of layer 3 can be from 45 to90 nm, and is preferably about 63 nm. When layer 3 includes asuperlattice of titanium oxide and silicon nitride, the layers in thesuperlattice can each have a thickness of from 1 to 45 nm.

Layer 4 is designed to reflect IR radiation. To accomplish this task,while retaining the possibility of a relatively neutral color inreflection, layer 4 is formed primarily from Ag. The Ag of layer 4 canhave a thickness in the range of about 8 to 24 nm, preferably 10 to 20nm, more preferably 12 to 18 nm, most preferably about 16 nm thick.

If the sum of the Ag thicknesses in layers 2 and 4 is less than about 16nm, insufficient infrared radiation will be reflected by the multilayercoating. If the sum of the Ag layer thicknesses in layers 2 and 4 ismore than about 40 nm, the visible transmission will be reduced tounacceptable levels.

Layer 5 serves to protect the multilayer coating of the invention fromscratches and abrasion; improves heat treatability of the coating; actsas a barrier to oxygen and other chemicals in the environment; andinfluences the optical properties of the low-emissivity coating.Preferably, layer 5 is silicon nitride. The silicon nitride can beSiN_(x), where x varies from greater than 0 to 1.34. The thickness ofthe silicon nitride of layer 5 is from 25 to 60 nm, and is preferablyabout 35 nm.

In embodiments of the present invention, layer 1 can include, inaddition to a sublayer of titanium oxide, a sublayer of silicon nitride,thus forming a superlattice of titanium oxide and silicon nitride. Theterm “superlattice” as used herein refers to any number of alternatingtitanium oxide and silicon nitride layers, including a titaniumoxide/silicon nitride bilayer. Suitable structures are shown in Tables2-3. The silicon nitride can enhance the barrier properties achievedusing titanium oxide and also influence the optical properties of thecoating when a sufficient thickness of silicon nitride is present. Inthe superlattice each of the titanium oxide sublayers and the siliconnitride sublayers can be from 1 to 30 nm thick.

TABLE 2 Sub-layer Material 1b silicon nitride 1a titanium oxide

TABLE 3 Sub-layer Material 1b titanium oxide 1a silicon nitride

In other embodiments of the present invention, the Ag of one or more oflayers 2 and 4 can include oxygen and/or nitrogen. The incorporation ofoxygen and/or nitrogen in the Ag improves the thermal and mechanicalstability of the Ag. The oxygen and/or nitrogen can be distributedhomogeneously throughout the Ag of a layer, or can be segregated to aportion of the Ag of a layer. The oxygen and/or nitrogen canincorporated into the Ag by adding oxygen and/or nitrogen to the inertgas used to sputter deposit the Ag. When the Ag including the oxygenand/or nitrogen is DC, AC or RF reactively sputtered, the amount ofoxygen and/or nitrogen in the inert gas can range from greater than 0 to20%.

In still other embodiments of the present invention, layers 2 and 4 caninclude, in addition to a sublayer of Ag, a sublayer of zinc oxide and asublayer of an oxidized metal. As shown in Tables 4-5, the zinc oxidesublayer serves as a substrate for the sublayer of Ag, and the sublayerof Ag serves as a substrate for the sublayer of an oxidized metal. Thesublayer of an oxidized metal protects the Ag from reactive materialssuch as oxygen in the environment.

TABLE 4 Sub-layer Material 4c oxidized metal 4b Ag 4a zinc oxide 3titanium oxide; silicon nitride; or superlattice of titanium oxide andsilicon nitride

TABLE 5 Sub-layer Material 2c oxidized metal 2b Ag 2a zinc oxide 1titanium oxide and/or silicon nitride layersThe sublayer of zinc oxide that can be in layers 2 and 4 is generallypolycrystalline. The zinc oxide can be ZnO. The present inventors havediscovered that, when deposited on amorphous titanium oxide, zinc oxideis particularly useful as a substrate for growing low sheet resistance,strongly adherent Ag layers. The amorphous titanium oxide, as discussedabove, provides an extremely smooth surface on which to grow subsequentlayers. The zinc oxide grows with the {0001} orientation, which orientsthe Ag to preferentially grow with a {111} orientation. The epitaxiallattice match between Ag {111} and ZnO {0001} leads to lower sheetresistance and improved adhesion of the Ag. The use of zinc oxide as asubstrate for Ag instead of another material lowers the Ag sheetresistance by approximately 1 Ω/□. The net result of using zinc oxide asa substrate for Ag is a decrease in emissivity without lowering thevisible, photopic transmission. The zinc oxide provides a means forforming a high conductivity, strongly adherent Ag layer with a thicknessas low as 8 nm.

Another interesting finding associated with the use of a sublayer ofzinc oxide is that the transmitted a* value increases by about one colorpoint (e.g., from a*=−3.0 to a*=−2.0) and the photopic transmissionincreases about +1.5% per zinc oxide layer added up to a maximumenhancement of up to +3%. These effects vary as the thickness of thezinc oxide changes.

In embodiments, the zinc oxide can include nitrogen and can berepresented by the formula ZnO_(x)N_(y). The nitrogen containing zincoxide can be formed by sputtering a Zn target in a sputtering gasincluding 33 to 84%, preferably 43 to 80%, O₂; 1 to 25%, preferably 3 to14%, N₂; and a remainder of argon. A coater manufactured by LeyboldSystems GmbH with model number Typ A 2540 Z 5 H/20-29 is suitable forsputter depositing the nitrogen containing zinc oxide, using gas flowsof 200 to 600 sccm, preferably 300 to 450 sccm O₂; 10 to 100 sccm,preferably 25 to 50 sccm N₂; and 100 to 300 sccm Ar. The addition ofnitrogen to the zinc oxide improves the thermal stability of the layeredcoatings of the present invention.

The sublayer of zinc oxide can have a thickness in the range of about 1to 20 nm, preferably about 6-7 nm. If the zinc oxide is too thick, thesheet restant of the Ag will begin to increase. By limiting zinc oxideunderlayer thickness to 20 nm and less, the zinc oxide allows for thedeposition of pinhole-free, low sheet resistance Ag films at lowerthicknesses than are possible with other substrates, while minimizingthe undesirable characteristic of thick zinc oxide. Because thin zincoxide enables thinner Ag films to be used, which enhances visibletransmission, use of thin zinc oxide leads to enhancements in thevisible transmission of low emissivity coatings.

The sublayer of oxidized metal in layers 2 and 4 protects the Agsublayer from corroding by acting as a diffusion barrier against oxygen,water and other reactive atmospheric gases. In addition, the sublayer ofoxidized metal improves adhesion between layers in the multilayercoating. Preferably, the sublayer of oxidized metal is an oxidized metalsuch as oxidized Ti, oxidized W, oxidized Nb, and oxidized Ni—Cr alloy.Different advantages and disadvantages are associated with each of thebarrier layers. Some of the barrier layers provide particularly highthermal and mechanical durability, while others particularly benefitcolor and/or photopic transmission and reflection. For example, an atleast partially oxidized Ni—Cr alloy (e.g., NiCrO_(y), where 0<y<2)provides particularly good heat treatability characteristics to amultilayer coating, enhancing the thermal and mechanical durability of acoating during heat treatments above 700° C., such as those necessaryfor bending and tempering a glass substrate. Preferably the oxidizedmetal is a suboxide near the metal insulator transition. Such a suboxidewill generally have an oxygen content less than the stoichiometricamount of oxygen in the fully oxidized metal. The suboxide will be ableto react with, and thus block diffusion of, additional oxygen and otherreactive gases. The oxidized metal sublayer can have a thickness in therange of 2 to 8 nm, more preferably 4 to 6 nm, most preferably about 5nm thick. The sublayer of oxidized metal is preferably formed byreactively sputtering a metal target in a sputtering gas including aninert gas and 10 to 75%, preferably 20 to 55%, oxygen.

In further embodiments of the present invention, the thermal andmechanical stability of various layers can be improved by dividing eachof the layers with a layer of the oxidized metal. The layer of oxidizedmetal strengthens the layers against thermally induced changes. Forexample, the titanium oxide and/or silicon nitride layers of layer 1 canbe divided by a layer of oxidized metal. In addition, at least one ofthe Ag sublayers in layers 2 and 4 can be divided by a layer of oxidizedmetal. Furthermore, at least a portion of the titanium oxide in layer 3can be divided by a layer of oxidized metal. Preferably, the layer ofoxidized metal is an at least partially oxidized Ni—Cr alloy (e.g.,NiCrO_(y), where 0<y<2). The oxidized metal provides improved mechanicalstability to the divided layers during heat treatments.

The layers in the multilayer coatings of the present invention can bedeposited by conventional physical and chemical vapor depositiontechniques. The details of these techniques are well known in the artand will not be repeated here. Suitable deposition techniques includesputtering methods. Suitable sputtering methods include DC sputtering,using metallic targets, and AC and RF sputtering, using metallic andnon-metallic targets. All can utilize magnetron sputtering. Thesputtering can be in an inert gas, or can be carried out reactively inreactive gas. The total gas pressure can be maintained in a range from5×10⁻⁴ to 8×10⁻² mbar, preferably from 1×10⁻³ to 1×10⁻² mbar. Sputteringvoltages can be in a range from 200 to 1200 V, preferably 250 to 1000 V.Dynamic deposition rates can be in a range of from 25 to 700nm-mM²/W-sec, preferably 30 to 700 nm-mm²/W-sec. Coaters manufactured byLeybold Systems GmbH with model numbers Typ A 2540 Z 5 H/13-22 and Typ A2540 Z 5 H/20-29 are suitable for sputter depositing the multilayercoatings of the present invention.

EXAMPLES

To further illustrate the invention, the following non-limiting examplesare provided:

Example 1

As discussed above, a sublayer of zinc oxide deposited on amorphoustitanium oxide promotes the wetting of Ag on the zinc oxide and theformation of thinner layers of pin-hole free Ag.

To demonstrate this, Ag films 16 nm thick were planar DC magnetronsputter deposited onto amorphous TiO_(x) (a-TiO_(x)) underlayers 25 nmthick, and also onto ZnO (5 nm)/a-TiO_(x) (25 nm) under(bi)layers.Transmission electron diffraction micrographs of the amorphous TiO_(x)showed only broad diffuse rings, indicating that the TiO_(x) wasamorphous. The ZnO and a-TiO_(x) dielectric layers were reactivelysputtered from metal targets. The substrates for the a-TiO_(x) layersincluded glass, and transmission electron microscopy (TEM) grids eachhaving a 50 nm thick, amorphous, silicon nitride, electron transparentmembrane peripherally supported by Si. The membrane was formed in amanner well known in the art by depositing silicon nitride by LPCVD(liquid phase chemical vapor deposition) onto a Si wafer, and thenback-etching the Si.

FIG. 1 shows bright field transmission electron micrographs comparing Agdeposited directly on the a-TiO_(x) with Ag deposited directly onto theZnO resting on a-TiO_(x). The Ag grown directly on the a-TiO_(x) has anabnormal microstructure with irregular grains. The Ag grown directly onthe ZnO has a more normal microstructure with regular grains. Theaverage normal grain size of the Ag directly on the ZnO is about 25 nm,while that of the Ag directly on the a-TiO_(x) is about 15 nm.

FIG. 2 shows dark field transmission electron negative micrographscomparing the Ag deposited directly on the a-TiO_(x) with the Agdeposited directly on the ZnO resting on TiO_(x). The dark field imageswere obtained using {220} Ag reflections. The images show that {111}oriented Ag grains giving rise to the strong 220 reflections have asignificantly larger average grain size (two to three times larger) whendeposited directly on the 5 nm thick ZnO than when deposited directly ona-TiO_(x).

FIG. 3 shows is a bright field transmission electron micrograph of Agdeposited directly the a-TiO_(x) underlayer near the center of the TEMgrid. The Ag film near the center of the TEM grid is clearlydiscontinuous. A grayish haze was observed by eye near the center of thegrid from the scattering of light from the rough surface. In contrast,the Ag film near the membrane supportive, back-etched Si was free ofpinholes and continuous. The discontinuous Ag film containing pinholesis believed to result from increased deposition temperatures at thecenter of the membrane due to thermal isolation. Remarkably, the Agdeposited directly on 5 nm thick ZnO was continuous over the entire TEMgrid, even in places where Ag deposited directly on a-TiO_(x) wasdiscontinuous.

The sheet resistance of the Ag films, measured when deposited onsubstrates of bulk glass, was found to be 5.68 Ω/□ with theZnO/a-TiO_(x) under(bi)layer and 7.56 Ω/□ with the a-TiO_(x),underlayer. Since there was no visual haze, and the films deposited onglass were on a heat sink even larger than the TEM grid edge, it isexpected that the Ag films were continuous and pinhole free on theglass.

Thus, zinc oxide provides an underlayer on which Ag preferentially growsas a pinhole free, continuous film. Furthermore, the sheet resistance ofthe Ag film can be reduced without an increase in Ag thickness. Theaddition of zinc oxide was observed to decrease the Ag sheet resistanceby approximately 1 Ω/□.

Example 2

A complex structure incorporating many of the features of the presentinvention appears in Table 6.

TABLE 6 Layer Material* 5 SiN_(x) 4c(2) NiCrO_(x) 4b(2) Ag 4c(1)NiCrO_(x) 4b(1) Ag 4a ZnO_(x) 3c TiO_(x) 3b NiCrO_(x) 3a TiO_(x),SiN_(x), or superlattice 2c(2) NiCrO_(x) 2b(2) Ag 2c(1) NiCrO_(x) 2b(1)Ag 2a ZnO_(x) 1a(2) TiO_(x) 1b NiCrO_(x) 1a(1) TiO_(x), SiN_(x), orsuperlattice 0 glass substrate *In Table 6, the subscript “x” indicatesboth stoichiometic and sub-stoichiometric compositions.

Various multilayer coatings including all, or a portion, of the layersshown in Table 6 were made by DC magnetron sputtering.

It was found that by varying the thicknesses of the silicon nitride andtitanium oxide layers the reflected color of the coating can bepositioned in any of the four color coordinate quadrants of the CIE 1976L*a*b* (CIELAB) and CIE 1931 Yxy (Chromaticity) color spaces. Techniquesand standards for quantifying the measurement of color are well known tothe skilled artisan and will not be repeated here.

FIGS. 4 a-4 c show transmitted, reflected glass side and reflected filmside color variance for the various multilayer coatings. As withconventional structures, color neutrality (colorless) was achieved withsome of the coatings. FIG. 4 a shows that the transmitted color varieddramatically in the second quadrant. FIGS. 4 b and 4 c show that thecoatings can produce reflected color in any of the four color coordinatequadrants of the CIE 1976 L*a*b* (CIELAB) and CIE 1931 Yxy(Chromaticity) color spaces.

The photopic transmission and reflection of the various coatings variedwith changes in the thickness of the silicon nitride and titanium oxide.The photopic transmission varied from about 50 to 80%. The reflectionfrom the glass side varied from about 5% to 22%. The reflection from thecoated side varies from about 3% to about 20%.

While the present invention has been described with respect to specificembodiments, it is not confined to the specific details set forth, butincludes various changes and modifications that may suggest themselvesto those skilled in the art, all falling within the scope of theinvention as defined by the following claims.

1. A low-emissivity coating on a substrate, the coating comprising, innumerical order outward from the substrate, a first layer including atleast one layer selected from titanium oxide layers and silicon nitridelayers; a second layer including Ag; a third layer including at leastone layer selected from titanium oxide layers and silicon nitridelayers; a fourth layer including Ag; and a fifth layer including siliconnitride, wherein at least one of the second layer and the fourth layercomprises a sublayer including Ag, and a sublayer consisting of a fullyoxidized Ni—Cr alloy directly on, and outward from the substrate from,the Ag of the sublayer including Ag.
 2. The coating according to claim1, wherein the first layer is from 5 to 30 nm thick.
 3. The coatingaccording to claim 1, wherein the first layer includes a titanium oxidelayer; and the titanium oxide in the first layer is amorphous.
 4. Thecoating according to claim 1, wherein the third layer comprises at leastone of a TiO₂ layer and a Si₃N₄ layer.
 5. The coating according to claim1, wherein the third layer comprises a superlattice of titanium oxideand silicon nitride.
 6. The coating according to claim 1, wherein atleast one of the second layer and the fourth layer consists of, innumerical order outward from the substrate, a first sublayer, whichincludes a zinc oxide; a second sublayer, which includes Ag; and a thirdsublayer, which includes a fully oxidized Ni—Cr alloy.
 7. The coatingaccording to claim 6, wherein the zinc oxide comprises nitrogen.
 8. Thecoating according to claim 1, wherein the Ag in at least one of thesecond layer and the fourth layer further comprises at least one ofoxygen and nitrogen.
 9. The coating according to claim 1, wherein the Agin at least one of the second layer and the fourth layer furthercomprises a means for strengthening the Ag against thermally inducedchanges.
 10. The coating according to claim 1, wherein at least one ofthe first layer, the second layer, the third layer, and the fourth layeris divided by a layer of an oxidized metal.
 11. The coating according toclaim 10, wherein the oxidized metal is an at least partially oxidizedNi—Cr alloy.
 12. The coating according to claim 1, wherein at least onelayer of the first layer, the second layer, the third layer, and thefourth layer is divided by a layer including a means for strengtheningthe at least one layer against thermally induced changes.
 13. Thecoating according to claim 1, wherein the silicon nitride comprises lessthan a stoichiometric amount of nitrogen.
 14. The coating according toclaim 1, wherein the first layer includes a layer of SiN_(x), where0<x≦1.34; the third layer includes a layer of SiN_(x), where 0<x≦1.34;and the fifth layer includes a layer of SiN_(x), where 0<x≦1.34.
 15. Thecoating according to claim 14, wherein the first layer includes a layerof SiN_(x), where x=1.34.
 16. A method of making a low-emissivitycoating on a substrate, the method comprising depositing at least onelayer including Ag on the substrate; and producing the coating ofclaim
 1. 17. The method according to claim 16, wherein the depositingcomprises sputtering.
 18. A method of making a low-emissivity coating ona substrate, the method comprising a step for depositing at least onelayer including Ag on the substrate; and producing the coating ofclaim
 1. 19. A low-emissivity coating on a substrate, the coatingcomprising, in numerical order outward from the substrate, a first layerincluding at least one layer selected from titanium oxide layers andsilicon nitride layers; a second layer including a first means forreflecting infrared radiation; a third layer having an index ofrefraction greater than or equal to 1.9 at a wavelength of 550 nm; afourth layer including a second means for reflecting infrared radiation;and a fifth layer including a means for protecting the coating fromabrasion, wherein at least one of the first means for reflectinginfrared radiation and the second means for reflecting infraredradiation comprises a sublayer including Ag, and a sublayer consistingof a fully oxidized Ni—Cr alloy directly on, and outward from thesubstrate from, the Ag of the sublayer including Ag.
 20. The coatingaccording to claim 19, wherein at least one of the first means forreflecting infrared radiation and the second means for reflectinginfrared radiation consists of, in numerical order outward from thesubstrate, a first sublayer, a second sublayer, which includes Ag, and athird sublayer, which includes a fully oxidized Ni—Cr alloy; and thefirst sublayer includes a means for preferentially orienting a crystalstructure of the second sublayer.
 21. The coating according to claim 19,wherein at least one of the first means for reflecting infraredradiation and the second means for reflecting infrared radiationconsists of, in numerical order outward from the substrate, a firstsublayer, a second sublayer including Ag, and a third sublayer; thefirst sublayer includes a zinc oxide; and the third sublayer includes ameans for preventing the Ag in the second sublayer from corroding. 22.The coating according to claim 21, wherein the zinc oxide comprisesnitrogen.
 23. A low-emissivity coating on a transparent substrate, thecoating comprising, in numerical order outward from the substrate, afirst layer including at least one layer selected from titanium oxidelayers and silicon nitride layers; a second layer including a zincoxide; a third layer including Ag; a fourth layer consisting of a firstfully oxidized Ni—Cr alloy directly on the Ag of the third layer; afifth layer including at least one layer selected from titanium oxidelayers and silicon nitride layers; a sixth layer including a zinc oxide;a seventh layer including Ag; an eighth layer consisting of a secondfully oxidized Ni—Cr alloy directly on the Ag of the seventh layer; anda ninth layer including silicon nitride.
 24. The coating according toclaim 22, wherein the first layer includes a layer of SiN_(x), where0<x≦1.34; the fifth layer includes a layer of SiN_(x), where 0<x≦1.34;and the ninth layer includes a layer of SiN_(x), where 0<x≦1.34.
 25. Thecoating according to claim 24, wherein the first layer includes a layerof SiN_(x), where x=1.34.